Estimation of radiogenic heat production rate of deep subsurface rock mass using natural gamma-ray logs
摘要
We estimated the radiogenic heat production rate of a deep subsurface rock mass using wireline logs from a borehole approximately 2 km deep. We first reviewed methodologies to determine the heat production of bedrock based on wireline geophysical logs from a borehole, as well as laboratory mass spectrometry analysis on rock samples. We then applied them to a test borehole of 1 km depth, where a comprehensive wireline logging campaign was conducted, including both spectral gamma-ray and relatively simple natural gamma-ray logging, valuable for estimating the radiogenic heat productions of rock formation. We calculated the heat production rates using the well logging data by applying a relationship between the generic heat production constants of major radioactive isotopes — uranium, thorium, and potassium — and their concentrations. We also used an empirical correlation to estimate the heat rates using total gamma-ray counts. The heat productions evaluated individually based on both spectral and total gamma in the tested borehole ranged from 1.45 to 6.59 µW/m3, corresponding to approximately 100–400 API in total gamma-ray counts. The average of heat rates estimated through the two gamma-ray logs of the rock mass is 2.12 ± 0.84 µW/m3. Although the heat rates evaluated from the two types of wireline logs were quite comparable across all depths, there were several intervals where the heat production magnitudes assessed from spectral-gamma data were somewhat higher. Laboratory mass spectrometry of rock samples retrieved from the same borehole was conducted for comparative purposes in estimating the heat production. For the 13 rock samples selected at various depths, heat production rates were determined with an average of 1.92 ± 0.54 µW/m3, closely aligning with those estimated from the logging data. We then attempted to extend and apply these approaches, reviewed in the test hole, to another deeper well reaching approximately 2200 m. In this deep borehole, we were limited to conducting only natural gamma-ray logging. For the 1500–2208 m depth interval, we measured total gamma emissions ranging from 11 to 116 API, which corresponds to estimated heat production rates of 0.16 to 1.83 µW/m3. Subsequently, we determined the concentration of radioactive elements by mass spectrometry analysis of rock samples retrieved from around 2200 m depth and assessed the heat production magnitude for verification. The average heat production rates in the rock samples, 0.55 ± 0.11 µW/m3, were in good agreement with those calculated from natural gamma-ray logs of adjacent depth intervals, approximately 0.52 ± 0.13 µW/m3.